>>1362156720 000 eggs doesn't mean 20 000 viable eggs able to be inseminated (also really they're "ovarian follicles which release eggs but it doesn't change the point). As time goes on, the eggs not just become fewer in number but also less viable (due to mutation load, cell death, etc.)
Apart from giving you an illustration of how a woman's chance of becoming a mother develops over time, you shouldn't look at the exact number of eggs, you should look at 1. the rates of fertility, and 2. the succes rates of carrying a pregnancy full term, and 3. the health of the child (e.g. birth defects), and look how these variables change with age.
